Recent mobile phones and other devices often use duplexers and dual filters as elastic wave devices including multiple radio-frequency filters with different passband.
FIG. 9 is a circuit diagram of existing elastic wave device 1.
Existing elastic wave device 1 is a dual filter including low-frequency filter 3 having passing characteristics in the low-frequency passband (relatively low frequencies); high-frequency filter 4 having passing characteristics in the high-frequency passband (relatively high frequencies); input terminals 5 and 6; and output terminals 7 and 8, all formed on piezoelectric substrate 2. Input terminal 5 is connected to low-frequency filter 3; input terminal 6, to high-frequency filter 4; and output terminals 7 and 8, to both of low-frequency filter 3 and high-frequency filter 4.
In low-frequency filter 3, input terminal 5 is connected in sequence to elastic wave resonator 9, elastic wave resonator 10, and output terminals 7 and 8. In high-frequency filter 4, input terminal 6 is connected in sequence to elastic wave resonator 11, elastic wave resonator 12, and output terminals 7 and 8. Here, all the elastic wave resonators shown in FIG. 9 are of longitudinally-coupled type.
Elastic wave resonator 10 includes multiple interdigital transducers (expressed as IDT, hereinafter) 15 between a pair of reflectors 13A and 13B; similarly, elastic wave resonator 12 includes multiple interdigital transducers (expressed as IDT, hereinafter) 16 between a pair of reflectors 14A and 14B.
In an existing elastic wave device, setting is made so that the reflection band (stop band) of reflector 13A will cover the low-frequency passband of low-frequency filter 3 and that the reflection band (stop band) of reflector 14A will cover the high-frequency passband of high-frequency filter 4.
In a passband, increasing the reflection of electric signals reduces the energy loss to decrease the insertion loss. In a stop band, decreasing the reflection of electric signals dissipates the electric energy to increase the attenuation.
A description is made of the concept of the passing characteristics of an existing elastic wave device referring to FIG. 10. Diagram (a) of FIG. 10 shows the insertion loss of low-frequency filter 3; (b), the reflectivity of reflector 13A of low-frequency filter 3; (c), the insertion loss of high-frequency filter 4; and (d), the reflectivity of reflector 14A of high-frequency filter 4. Examples of prior art documents include following patent literature 1 for instance.
In the existing elastic wave device, low-frequency filter 3 and high-frequency filter 4 are connected to common output terminals 7 and 8. Consequently, when low-frequency filter 3 outputs electric signals in the low-frequency passband to output terminals 7 and 8, some of the signals result in reaching high-frequency filter 4 as well. At this moment, the energy of the signals is dissipated through reflectors 14A and 14B of high-frequency filter 4, causing the loss of electric energy. Hence, the existing elastic wave device undesirably involves high insertion loss at low-frequency filter 3.
In the same way, when high-frequency filter 4 outputs electric signals in the high-frequency passband to output terminals 7 and 8, some of the signals result in reaching low-frequency filter 3 as well. At this moment, the energy of the signals is dissipated through reflectors 13A and 13B of low-frequency filter 3, causing the loss of electric energy. Hence, the existing elastic wave device undesirably involves high insertion loss at high-frequency filter 4.